Electric discharge apparatus



Nov. 30, 193 7.

ELECTRIC DISCHARGE APPARATUS Filed July 5, 1935 ,0 max/mum ,0 ,0

INVENTOR BYWQHQI Dalkznloach XB QQ Q I'HS ATTORNEYS w. DALLENBACH 2,100,797

Patented Nov. 30, 1937 UNITED STATES PATENT OFFICE Germany Application July 3, 1935, Serial No. 29,744 In Germany July 24, 1934 4 Claims.

The invention relates to electric discharge vessels more particularly mercury vapour rectifiers and other current converting apparatus. In such electric discharge vessels it is already known to arrange multi-channel tubular bafiles in front of the anodes in the anode protecting tubes in order to increase the load which can be applied to gas or vapour filled vacuum discharge apparatus without giving rise to the danger of striking back or high frequency oscillations.

The present invention represents a further development of electric discharge vessels of this kind and resides in imparting electrical characteristics affording a safeguard against striking back and high frequency oscillations in that the pressure in the vessel is constantly maintained above the limit at which the saturation current per channel of the multi-channel tubular baflle, which current is dependent upon the pressure, attains its maximum Value. By suitably adapting the length of the baflle channels and the magnitude of the pressure to one another as high a load as possible on the anode can be attained.

The invention will be described more fully with reference to the accompanying drawing in which:

Fig. 1 shows one embodiment of a rectifier according to the invention by way of example.

Fig. 2 shows an enlarged plan view of the multi-channel tubular battle, and

Fig. 3 shows graphically the relationship between the saturation current and the gas pressure.

I is the vacuum vessel of a mercury vapour rectifier, 2 is the cathode and 3 is one of the anodes introduced by means of an insulator 4 and surrounded by an anode protecting tube 5. Inserted in this anode protecting tube is the multi-channel tubular baffle 6 which as shown in Fig. 2 may be of honeycomb form. Naturally the channels in the baffie can have any other cross sectional form and the baffle can be constructed in the form of a simple grating. I is an observation aperture through which the bafile 6 can be observed. 8 is the condensation chamber which, in the same way as the rectifier vessel, is surrounded by cooling chambers 9. I0 indicates a protecting tube inserted in the cathode.

Calculation and experiment have shown that a definite relationship must be maintained between the length L corresponding to the distance of the lower end of the baffle from the anode surface and the gas or vapour pressure within the vacuum vessel I for a given magnitude of the current which can pass to the anode without striking back occurring. This results from the following:

Assuming in the first place that the gas pressure can be kept constant at a definite value, in that for example the temperature of the vacuum vessel and more particularly the temperature of the walls which determines the mercury vapour pressure in the anode space, remains constant or a neutral gas, e. g., a rare gas, is introduced into the vacuum discharge vessel and is maintained at a constant pressure. If now the surface of the bafiie is observed through the observation aperture land if the current flowing to the anode 3 is varied from small to high values then it is found that at small values of the current passing to the anode only one of the many channels in the baflle 6 is utilized. On gradual increase in the strength of the current the discharge suddenly extends to two channels at a definite value of the current i and the discharge remains limited to these two channels until the same critical value of the current is reached for each of the two channels as previously for one channel. On the further increase of the strength of the current, three channels carry the discharge and this continues until the same critical value 2' is obtained for each of these channels as in the previous cases when one or two channels carry the discharge i. e., in other words the number of channels carrying the discharge is proportional to the total current passing to the anode and the total current 1:11.. i. In this formula 11. is the number of channels carrying the discharge, 2' the maximum current per channel and I is the total current.

On further increase in the total current the condition is attained at which all the baffle channels are carrying the discharge. On further increase of the load on the anode above this value of the,current,.a current larger than the critical value 2 must be carried by each baffle channel.

The manner in which this critical value of the current depends upon the gas pressure is of great importance. It has been found that this dependence follows the curve shown in Fig. 3. This curve shows that at small gas pressures, the capacity of a channel of the baffle 6 as expressed in terms of the current i is small and increases as the gas pressure increases. At a certain gas pressure a maximum value is obtained and thereafter there is a decrease. These observations can be explained as follows:

Due to the intensive ionization of the gas occurring at the lower aperture of the baffle there occurs a so-called plasma in the interior of the baffle in which electrons diffuse towards the region of the anode surface adjacent the baille channels in question. At low gas pressure the diifusion of the electrons is very little obstructed by the gas. On increase in the strength of the current however there is a depletion of ions in the interior of the baffle channels carrying the current and thus an increased voltage over the baffle channel. At this instant, a second balile channel comes into use which assists to carry the current until the saturation current strength is again reached in both channels so that on further increase in the strength of the current,

a third channel comes into use, then a fourth, and so on. If now the gas pressure is increased, then clearly the saturation current can be increased until the gas cushion existing within a channel prevents the electrons from difiusing from the front of the channel to the anode surface. As soon as this obstruction to the diffusion becomes appreciable, a further increase in the gas pressure does not result in an increase in the saturation current but the possible saturation current through one bafile channel decreases, i. e., the downward slope of the curve shown in Fig. 3 is reached.

In order that the current passing to an anode should be distributed uniformly over all the baffle channels of the anode in question and thus result in a uniform heating of this anode, it is necessary that the larger currents occurring in operation in excess of the current 1; which has a maximum as shown in Fig. 3, should be in dependence upon the gas pressure. As however on the other hand, it is necessary to keep away from the limit at which a depletion of ions occurs in order to avoid excess voltages, high frequency oscillations and resultant striking back, it follows that the gas pressure of the apparatus must invariably be maintained at a value which lies above the limit at which the saturation current per bafile channel attains its maximum value, i. e., it is necessary to operate in the pressure range corresponding to thedownward slope of the curve. .Afavourable value of this. gas pressure for example is afforded by the value P=p1 in Fig. 3, because at this pressure the current carried by the anode can be increased to a value I1=i1.N where i1 is the saturation current per bafiie channel and N is the total number of bafile channels per anode. On attaining this current I1, all baflie channels of the anode in question participate in the discharge. If the current passing to the anode is increased above this value I1, then the baille channels are compelled to carry a current in excess of their saturation value. No disadvantage occurs when this happens because the voltage drop in the arc in the interior of the bafile channel increases extremely slowly with the current strength and in any case increases only until there is a depletion of ions in the interior of .all bafile channels. This moment is attained at a current strength I2=Ni2 where in is given by the extension of the rising portion of the curve Fig. 3, and represents the limit at which the depletion of ions commences in each bafile channel. The reason why this curve represents the extension of the rising portion of the curve in Fig. 3 is that the depletion of ions occurring per baflle channel determines the saturation current per bafile channel in the rising portion of the curve of Fig. 3.

In practical operation of the rectifier, the pressure P1 will be so chosen that I2=i2.N indicates the largest current which occurs in practice without disturbances being permissible. It

is also advantageous to select the gas pressure P1 at so high a value that at relatively small load on the anode the anode current is distributed uniformly over all the baffle channels i. e., the current strength I1 is relatively small in comparison with the normal load on the apparatus and also in comparison with the maximum value of the current given'by the maximum of the curve'in Fig. 3.

' If by way of concrete example 30 mm. is taken as the length of the bafiie channels i. e., the lower aperture of the bafile channels is 30 mm. from the surface of the anode, then it is found that the favourable gas pressure is attained if themean free'path of the electrons in the gas, mercury vapour for instance or any inert gas, e. g., a rare gas (helium, neon, xenon, argon or krypton) is of the order of 1 mm., i. e., the length of the bafile channels is about 30 times the electron path in the gas or vapour. With mercury vapour this gives a gas pressure of the order of magnitude of 0.1-0.15 mm. of mercury while with rare gases similar figures are obtained corresponding to the altered mean free paths.

1 V For operating the apparatus it is not necessary for a favourable gas pressure to be maintained precisely. It is sumcient if, as already mentioned, the critical value of the gas pressure at which the saturation current per bafile channel attains its maximum is in no circumstances fallen short of. The result of this is that in apparatus operated with pure mercury vapour, the vessel must be maintained at so high a temperature by artificial heating that the mercury vapour pressure within the vessel, more particularly in the vicinity of the anodes, is above the critical value determined from Fig. 3. In apparatus with an auxiliary filling of rare gas, the rare gas filling is so chosen andif necessary maintained at such a value, by automatic devices introducing the same for example, that its gas pressure corresponds to the rising portion of thecurve, Fig. 3. At high load, the rare gas mixes with the mercury vapour and the total pressure within the apparatus increases further. No disadvantage is involved. On the contrary, with an increasing proportion of rare gas in the mercury vapour, the value of the current at which a depletion of ions occurs is further increased.

The value of the gas pressure in question can be directly determined by experiment by altering the gas pressure and the current passing to the anode, as well as the temperature of the vessel if desired, and observing the baflie channels through the observation aperture 1 until the desired value is determined. When this has been done the observation aperture 1 can be permanently sealed. It should be pointed out that the adjustment is necessary only in the first construction of a new type of rectifier.

Experiments which have been made have also shown that the gas pressure at which the maximum saturation current occurs as shown in Fig.

3, is dependent upon the length of the bafile channels, i. e., upon the distance of the end of the bafiie from the surface of the anode, the relationship between this distance and the gas pressure being such that the corresponding gas pressure increases with decrease in the length L. By reducing-the length L of the baflle channels and correspondingly increasing the gas pressure, the load which can be carried by the anode can be still'further increased until the limit is attained which for example is determined by the thermal load which can be applied to the anode surface.

What I claim is:

1. An electric discharge vessel comprising a mercury cathode, at least one anode, an anode protecting tube surrounding each anode, a multichannel bafile disposed within each protecting tube in the path of the discharge and terminating at its outer extremity 30 mm. from the corresponding anode and a mercury vapour atmosphere at a pressure lying in the range between 0.1-0.15 mm. of mercury.

2. An electric discharge vessel comprising a mercury cathode, at least one anode, anode protecting means surrounding each anode, nonpolarized multi-channel baflle means associated with each protecting means in the path of the discharge so as to create a uniform firing over the entire firing surface of said anode, and an inert gas atmosphere at a pressure above the limit corresponding to the maximum value of the saturation current per baflle channel.

3. An electric discharge vessel comprising a cathode, at, least one anode, anode protecting means surrounding each anode, multi-channel bafiie means associated with each protecting means and slightly spaced from said anode so as to create a uniform firing over the entire firing surface of said anode, said baflie means being interposed between said anode and the cathode, and a gaseous atmosphere at a pressure above the limit corresponding to the maximum value of the saturation current per baflie channel.

4. An electric discharge vessel comprising a cathode, at least one anode, anode protecting means surrounding each anode, multi-channel bafiie means associated with each protecting means and slightly spaced from said anode so as to create a uniform firing over the entire firing surface of said anode, said baffle means being interposed between said anode and the cathode, and a mercury vapor atmosphere at a pressure above the limit corresponding to the maximum value of the saturation current per baflie channel.

WALTER DALLENBACH. 

